Moonlight triggers the synchronized spawning of reef-building corals; however, the mechanism underlying detection of moonlight by these animals is largely unknown. Levy et al. now demonstrate the presence of ancient blue-light-sensing photoreceptors, cryptochromes, in the reef-building coral Acropora millepora (phylum Cnidaria). Cryptochromes regulate entrainment of the circadian clock of higher animals and plants. Expression of two coral cryptochrome genes, cry1 and cry2, was rhythmic under a light-dark cycle but not in constant darkness. Expression of cry2 varied with the full moon. This work suggests that cryptochromes not only function in the circadian clock of plants and higher animals but also may trigger the synchronized spawning of the Great Barrier Reef.